A Mechano-Acoustic Indentor System for In-vivo Measurement of Non-Linear Elastic Properties of Soft Tissue

Objectives: Soft tissue exhibits non-linear stress-strain behavior under compression. Characterizing its nonlinear elasticity may aid detection, diagnosis, and treatment of soft tissue abnormality. The purposes of this study were to develop a rate-controlled mechano-acoustic indentor system and a corresponding finite element optimization method to extract non-linear elastic parameters of soft tissue and evaluate its test-retest reliability. Methods: Eleven volunteers (7 men, 4 women) were recruited from the student and staff population at a chiropractic college. An indentor system using a linear actuator to drive a force-sensitive probe with a tip-mounted ultrasound transducer was developed. Twenty independent sites at upper lateral quadrant of buttock from 11 asymptomatic subjects were indented at 6%/s for 3 sessions, each consisting of 5 trials. Tissue thickness (T), force at 25% deformation (Aexp), and area under the load-deformation curve from 0 to 25% deformation (Fexp) were calculated. Optimized hyperelastic parameters of the soft tissue were calculated with a finite element model using an order-one Ogden material model. Load-deformation response on a standardized block was then simulated and the corresponding area and force parameters (AFE, FFE) were calculated. Between-trials repeatability and test-retest reliability of each parameter was evaluated using coefficients of variation (CV) and intraclass correlation coefficients (ICCs) respectively. Results: Load-deformation responses were highly reproducible under repeated measurements. CVs of variation of T, Aexp and Fexp averaged 0.51%, 2.31%, and 2.23% respectively. ICCs ranged between 0.959 and 0.999, indicating excellent test-retest reliability. Conclusions: The automated mechano-acoustic indentor system and its corresponding optimization technique offers a viable technology to make in vivo measurement of the non-linear elastic properties of soft tissue. This technology demonstrated excellent between-trials repeatability and test-retest reliability with potential to quantify the effects of a wide variety of manual therapy techniques on the soft tissue elastic properties.